1. Field of Invention
The present invention relates to a semiconductor structure. More particularly, the present invention relates to a varactor structure.
2. Description of Related Art
In a typical communication system, information signals, such as television program, will be tuned and placed on high frequency carrier waves to facilitate transmission of signals. Lots of information signals are transmitted simultaneously due to the properties of different frequencies having different carrier wave signals. Therefore, a receiver in a communication system is required to use a voltage controlled oscillator (VCO) to separate information signals from the carrier waves. In a VCO, a LC (inductance-capacitance) circuit comprised of the varactor and the inductance is included. The oscillation frequency of the VCO is modified by the properties of the capacitance of the varactor being modified according to the tuning of the voltage.
Commonly seen varactors include a MOS varactor with metal-oxide semiconductor transistor structure, and a Junction varactor with p-type doped regions and n-type doped regions arranged alternatively. Although the MOS varactor has a feature of a large range of tuning ((the maximum capacitance to the minimum capacitance)/the minimum capacitance), the capacitance is determined by the ratio of the upper electrode (i.e., the gate of the MOS varactor) and the sum of area of the bottom electrode between the both sides of the upper electrode in the substrate (i.e., the source area/the drain area of the MOS varactor), that is the unit capacitance is direct proportion to the ratio of the area of the upper electrode to both sides of the bottom electrode. Accordingly, how to increase the ratio of the area of the upper electrode to the sum of the area of the bottom electrode to improve the unit capacitance becomes an important issue of current development of the varactor.